Slab-on-Ground Design

by Ken BondyUsing the PTI

Method

Biggest Single U.S. Market for Post-Tensioning Tendons

Over 40%Over 40% of all P/T tendons sold in the USA are for residential foundations.58,000 tons58,000 tons of tendons installed in residential foundations in the year 2000 alone.Represents about 220,000 homes in one year.

HistoryFirst used the late 1960’s in Texas and CaliforniaWidest usage has been in

Texas (50%)California (32%)Nevada (7%) Louisiana (5%)

Usage increasing (4%) inArizonaColorado FloridaGeorgia

Evolution of Design MethodsEarliest methods (±1965) were semi-empirical

Based upon simplified mathematical modelsAssumed loss of support (Spanability)

Confirmed by actual performancePTI Method (1980) based upon rigorous mathematical study of soil/structure interaction

Most comprehensive design method ever developed for behavior of concrete foundations on expansive soils

The PTI Design MethodThe PTI Design MethodBased upon a finite element computer model of soil/structure interaction, with research sponsored by PTI and executed at Texas A & M University in late 1970’s1st Edition published in 1980, 2nd Edition in 1996Incorporated into model building codes (UBC 1997, IBC 2000)Used to design millions of existing foundations

PTI Publications

Technical Information Available

Structural Function of P/T FoundationActs as a buffer between the soil below and the superstructure above to prevent unacceptable deformations in superstructure.Foundation is designed to resist or span over moisture-induced deformations in the soil below, while still maintaining its top surface within permissible level tolerances.

Expansive Soil Swell Modes

Edge LiftSoils are wetter at slab edge than at any point inside slab edge.

Center LiftSoils are drier at slab edge than at any point inside slab edge.

Soil Model

Edge Lift

Center Lift (Edge Drop)

Geotechnical Engineer Provides Critical Soil Design Parameters

Edge Moisture Variation Distance eemmThornthwaite Moisture Index (climate)Soil PermeabilityVegetation

Unrestrained Differential Swell yymmProperties (activity) of clayDepth of clay (active zone)Soil suction

One set of eemm & yymm values established for each swell mode (edge and center lift)Design cannot be done without these parameters

3.0 in0.75 inyymm

5.0 ft2.0 fteemm

Center Lift

Edge Lift

New Method For Determining em & ym

Under review by PTI Slab-on-Ground Committee.em based upon soil properties:

Unsaturated diffusion coefficient αSoil suction.Soil permeability.Cracks and roots.

Soil Fabric Factor (roots, cracks, layers).

Simplified method for determining ym based primarily on soil suction profiles.Considers effect of vertical barriers (cutoff walls).

Edge Lift

All Structural Activity

∆

ym

Wet to Dry

Center Lift

All Structural Activity

∆

ym

Dry to Wet

Ribbed and Uniform Thickness Foundations

PTI Design Method based on “ribbed” foundation system

Slab thickness t=4” minimumRibs=Grade Beams

h=t+7” with 12” minimum depth.b=8-14”

Rib spacing S=6’ minimum, 17’ maximum.Can be converted to uniform (solid) thickness slab

Overlapping Rectangles

Determine preliminary geometry and layout:

Rib spacingRib sizeSlab thickness

Divide slab into overlapping rectangles congruent with slab perimeter.

Rectangle A

24’ x 42’

Rectangle B

16’ x 36’

Design EquationsFor each swell mode (edge or center lift)

For each direction (Long or Short), use equations to determine:

Maximum MomentMaximum ShearMaximum Differential deflection

Design Based on Uncracked Section

Effects of cracking studied in detail in original research and subsequent publications available through PTI.Effects of cracking generally inconsequential due to

Location of shrinkage cracks.Increased soil support after flexural cracking.

Allowable Concrete StressesFlexural

Tension 6√f’cCompression 0.45f’c

Shearvc = = 1.7√f’c + 0.2fp

Differential deflectionL/C∆

L=smaller of total slab length or 6β.C∆ = coefficient based on superstructure material.

FlexuralTension 6√f’cCompression 0.45f’c

Shearvc = = 1.7√f’c + 0.2fp

Differential deflectionL/C∆

L=smaller of total slab length or 6β.C∆ = coefficient based on superstructure material.

Typical Slab Layout

Typical Details

Ribbed Foundation

Uniform Thickness Foundation

Thank You!